Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to a switched cushion stop for a variable cam timing phaser of a variable cam timing system.
Description of Related Art
Internal combustion engines have employed various mechanisms to vary the relative timing between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more “vane phasers” on the engine camshaft (or camshafts, in a multiple-camshaft engine). Vane phasers have a rotor with one or more vanes, mounted to the end of the camshaft, surrounded by a housing assembly with the vane chambers into which the vanes fit. It is possible to have the vanes mounted to the housing assembly, and the chambers in the rotor assembly, as well. The housing's outer circumference forms the sprocket, pulley or gear accepting drive force through a chain, belt, or gears, usually from the crankshaft, or possibly from another camshaft in a multiple-cam engine.
Apart from the camshaft torque actuated (CTA) variable camshaft timing (VCT) systems, the majority of hydraulic VCT systems operate under two principles, oil pressure actuation (OPA) or torsional assist (TA). In the oil pressure actuated VCT systems, an oil control valve (OCV) directs engine oil pressure to one working chamber in the VCT phaser while simultaneously venting the opposing working chamber defined by the housing assembly, the rotor assembly, and the vane. This creates a pressure differential across one or more of the vanes to hydraulically push the VCT phaser in one direction or the other. Neutralizing or moving the valve to a null position puts equal pressure on opposite sides of the vane and holds the phaser in any intermediate position. If the phaser is moving in a direction such that valves will open or close sooner, the phaser is said to be advancing and if the phaser is moving in a direction such that valves will open or close later, the phaser is said to be retarding.
The torsional assist (TA) systems operates under a similar principle with the exception that it has one or more check valves to prevent the VCT phaser from moving in a direction opposite than being commanded, should it incur an opposing force such as a torque impulse caused by cam operation.
The motion of the rotor assembly relative to the housing assembly can be halted by a lock pin, which in a locked position locks the rotor assembly to the housing assembly by being received in both the rotor assembly and the housing assembly. In an unlocked position, the lock pin only is received by the rotor assembly. It should be noted that the lock pin can also be received by the housing assembly and engage the rotor assembly to lock the relative motion of the rotor assembly to the housing assembly.
When the phaser is moving towards a position in which a lock pin can engage a recess of the housing assembly, the lock pin can miss the recess of the housing assembly and the vane can hit the chamber wall with a force that causes significant noise. This misalignment of the lock pin with the recess of the housing assembly and the noise associated with the vane hitting the chamber wall can be detrimental to phaser performance.